Exposure meter instrument



May 3.0, 1950 c. F. TAYLOR r AL 2,509,893

EXosURE METER INSTRUMENT Filed Aug. 26, 1947 Inventors:

Clement F? Taylor, Clyde W. Mc (Darty, 4Allen G. Stmson,

Their Attorney emma mylar), reso axrosmrz Maren ms'rammn'r clment r. Taylor, Peabody, Clyde w. monty, Swampscott, an Allen G. Stinson, Lynneld,

Mass., assignors General Electric Company.

a corporation of New York Application August 26, 1947, Serial No. 770,6 claims. (c1. 1v1-95) Our invention relates primarily to an electrical measuring instrument intended for use in a small sized exposure meter, and concerns features of l construction and operation winch permitof obsure or light meter.

. In carrying our invention into effect, we employ a permanent magnet eld instrument with the permanent magnet inside the moving armature coil, and an external U-shaped magnetic re'- turn strap instead of the more conventional reverse arrangement. This enables us to obtain the necessary small diameter instrument for our purposes having the necessary high sensitivity. The magnetic circuit air gap is shaped to obtain a logarithmic open scale distribution desirable in exposure meters. We employ internal pivots and lead-in spiral return springs inside the arma--` ture coil instead of the more conventional arrangement where the lead-in spirals are outside the amature coil.. This enables us to obtain the required length of coil without increasing the axial length of the complete instrument beyond the length of the armature coil. The instrument pointer is likewise secured to the armature inside the coil and swings in an arc the plane of which cuts the armature coil. Hence the maximum axial length of the instrument is the length of the armature coil which is made no longer than is necessary to obtain `the required sensitivity in a well designed instrument. The internal permanent magnet is secured in xed relation to the external ux return strap by a U-shaped strap which also acts as a spring and is used for adjust, ing the clearance between the jewels and internal pivots of the armature. The construction while small in size and high in sensitivity is nevertheless suiiiciently rugged as to permit of the rough 2 meter and instrument taken on line 2-2 of Fig. 1, and Fig. 3 is a perspective view of the magnetic circuit and* amature coil assembly with the two axially separated for the sake of clarity of illustration.

,The permanent magnet eld for the instrument is supplied by a generally oblong-shaped salient pole permanent magnet I which is placed inside the armature and polarized across its long or major axis as indicated by the N and S designations in Figs. 1 and 3. The main pole piece sections which are part of the permanent magnet have an axial length somewhat longer than the central portion of the magnet. The permanent magnet pole pieces are not exactly symmetrical; thus the N pole piece has a main elongated portion 2 and a shorter upscale extending portion 3. The permanent magnet shape is designed in relation to the outer flux return U-shaped magnetic part l to obtain the desired flux distribution. The U- shaped outer magnetic part 4 is secured in iixed relation to the internal permanent magnet l by a U-shaped nonmagnetic strap 5 which may be made of brass. The base part of the U-shaped part 5 is welded to the inner central part of magnetic return part 4, and its lower limb is secured to the lower central portion of permanent magnet l by a rivet E (see Fig. 2) which extends upwardly through an axial hole in the permanent magnet I handling incident to being carried around in a instrument embodying our invention. Fig. 2 is a cross-sectional view' through such an exposure and is peened over at the upper surface at 1, thus rigidly securing the lower limb of strap 5 to the magnet and to the flux return strap 5.

The upper limb of strap 5 is free of the magnet and supports the upper jewel il, while the lower jewel 9 is supported in the lower end of rivet 6. A screw III extends through a hole in the upper limb of strap 5 and is threaded into the lower limb thereof at a point between magnet l and magnetic return part l. The two limbs of 5 are originally bent to have a tendency to spread apart a greater distance than the required distance between the pivots 8 and 9, and this spreading is restricted and is adjustable by the screw ill which thus Yserves as the jewel spacing adjustment. Preferably, the upper limb of the part 5 is weakened as by an opening at Il near part l, so that the bending caused by adjustment of screw 5 comes at point Il, and hence the pivot adjustment is substantially in an axial direction, and proper axial alignment of the jewels is obtained when the adjustment is correct. It is to be noted that the strap 5 embraces the magnet I at the reduced axial length central portion thereof, and that the upper surface of the permanent magnet has an additional recess at l2 to accommodate the upper cross-sectional area to the point where it would be oi' an ineillclent design. v u 4The amature coil I4 with itsdnternal pivots lland Il is rotatively supported in the jewels .and l, with the coil sides movable in the air npbetween the inner permanent magnet I and outer tux return. member 4, as represented in Pigs. 1 and 2. The lower lead-ln spiral I'l-is fastened at its inner end to the inner side of the coil at the button which supports lower pivot il. The outer -end of this spiral i1 is secured to an arm Il which has a hub portion pivoted about rivet l between the lower limb of strap l and a collar on the rivet. Arm Il is engaged by a forked lever l! secured to and rotatable with a aero set pin 2l rotatively mounted through the bottom wall portion 2l of the casing employed. The pin 2t is friction tight but may be rotated with a screw driver from outside the casing to adjust the tension of lower spiral Il and thus the xero position of the amature pointer 22 on the instrument scale 23. The inner end of the upper spiral 24 is secured to the inner side of coil I4 at the button which supports the upper pivot I5, and the outer end of the spiral is secured by a strap 25 xed to the casing structure as by molding. It is thus seen that the jewels, pivots,

um from um me indicating position. it am moves intoan abruptly increasing strength of field ilux across the air gap to about l5 degrees from the zero endl and then into a much more` l salient pole internal permanent magnet and the and lead-in spirals of the armature assembly are inside the length of the coil, and that the total axial length of the instrument corresponds to the total length of the coil.

The casing may be made out of some molded insulating compound such as plastic, hard rubber,

etc. As represented, it is made in two sections, a cover section 20 and the base section 2 I. 'I'hese sections are iltted together at a Joint 3l and after assembly of the instrument in the casing. These sections may be secured together by screws or cement.

The casing contains a light cell at l2 may be exposedV to light through a transparent window glass t3 and corresponding upper end opening in the casing. The window glass is held in place against the inside wall ,of the casing over the window opening by a resilient wire spring part 34 inserted between the glass 33 land a ledge 3l molded into the casing structure. An analogous arrangement secures the cell 32 in place. The light cell 32 is electrically connected across the armature coil I4 through the spirals l1 and 24, so that when the cell is exposed to light through window glass 33, current ows in lcoil I4 to cause the instrument to deflect upscale in,

proportion to the light value. The electrical connections are not completely shown.

To the front of the light cell and window therefor is the scale plate 35 having the scale 23 on its front surface, and over which the instrument pointer 22 is adapted to swing. 'I'he scale and pointer are exposed to'view through a front glassjections 38 at its ends which fit tightly into corresponding recesses molded into the base section of the casing. The strap 4 preferably is sprung which gradual increase in air gap opposite thereto over the remaining range of deflection of the instrument. The total angley of deflection provided for is approximately 55 degrees. 'Ihe arrangement provides for and produces an open, uniformly graduated logarithmic scale as shown. It is to be noted that the oifset portion I of the perma-` nent magnet occurs at only one salientl pole and not the other. The outer flux return strap 4 is of U shape and vdoes not extend completely around the air gap, and there is little leakage flux in the air gap area where the strip U is absent. y

A successful instrument of this character has been built having an armature coil approximateb net material, such as the material described in United States Patent No. 2,295,082, September 8, 1942, to Jonas. Placing the lead-ln spirals, which also serve as zero return springs, inside the armature instead of outside, -permits of the reduced axial length of the armature illustrated, and at the same time allows for a satisfactory axial spacing between the internal pivots. If these pivots are placed too close together, the rotary system lacks stability and pointer flop results.

The complete instrument and casing as built has a depth of inch, a width of in, inches, and a length of 1H inches, maximum dimensions. This small size is obtained with a logarithmic scale instrument which is both rugged in construction and of the required sensitivity, having the structural features described.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. An electrical measuring instrument comprising an internal oblong-shaped permanent inwardly very slightly in inserting it into place. 15 magnet having integral salient pole pieces of opposite magnetic polarity at the extremities of its major axis, an external U-shaped magnetic return strap for said permanent magnet with its limbs spaced from and opposite the pole pieces of said magnet to provide an armature'air gap having its minimum length and maximum ux density areas opposite said pole pieces, nonmagnetic means secured between the central portions of said magnet and return strap for securing said parts in fixed relation, an armature coil pivoted centrally of said magnet with its coil sides in said air gap, said coil having a range of rotation from below to above the concentrated ilux areas of said air gap, one of the pole pieces of said magnet having an unsymmetrical extension in the upscale direction of rotation of said coil tov Provide an air gap area of gradually diminishing flux strength for the coil as it rotates upscale from the concentrated flux air gap areas.

2. An electrical measuring instrument comprising an internal permanent magnet of oblong shape with integral pole pieces of opposite magnetic polarity at the extremities of its major axis, an external U-shaped strap of magnetic material with its limbs spaced from the pole pieces of the magnet to provide a iiux return path therefor, a U-shaped strap of nonmagnetic material secured at its base to the inner side of the base of the U-shaped magnetic return strap and straddling the central portion of the permanent magnet, with one limb of said nonmagnetic part secured to said magnet and the other limb free, an armature coil having an axis of rotation centrally of said magnet and passing through the limbs of said nonmagnetic strap and'with its coil sides in the air gap between the permanent magnet and flux return strap, internal jewels for said coil on opposite sides of said permanent coil pivoted centrally of the internal part with its coilsides rotatable in said air gap, a U-shaped strap of nonmagnetic material secured at its base to the external magnetic part and with its limbs straddling the internal magnetic part on the axis of rotation of said coil, one limb being secured to said internal magnetic part and the other limb being free, internal Jewels foxl piv oting said coil at opposite ends of said internal part one jewel being secured to said internal part and the other being secured in the free limb of said nonmagnetic part, said free limb being made nexible near the point where secured to Y Number said external part for the purpose oi adjusting the spacing between said jewels, and a screw extending between said limbs for adjusting such spacing.

4. An electrical measuring instrument comprising an internal permanent magnet of oblong shape and having integral salient pole pieces of opposite magnetic polarity at the extremities of its major axis, an external U-shaped magnetic strap forming a magnetic return path for the flux of said magnet, the limbs of the strap being spaced from the poles of the magnet to form an armature air gap, an armature coil pivoted to rotate in said gap, a central opening in the permanent magnet on the axis of rotation oi said coil, a U-shaped member of nonmagnetic material secured at its base to the base of said magnetic return strap and with its limbs straddling the permanent magnet within and on 'the axis of rotation of said coil, a rivet extending through the 'opening in said permanent magnet and one of the limbs of said nonmagnetic member for securing the permanent magnet and. magnetic strap in fixed relation, internal pivot jewels for said coil, one of which is located in the. end oi said rivet and the other of which is carried by the other limb of said nonmagnetic member,V and means for adjustably flexing the last--mentioned limb to adjust the spacing between said jewels.

5. An electrical measuring instrument com prising an internal permanent magnet and an external magnetic return core, said parts being separated by an armature air gap, an armature coil surrounding the permanent magnet and pivoted on an ax'is passing therethrough, said coil having a diameter which is greater than its axial length, internal pivots for said armature, leadin spirals for said armature within the coil. and a pointer for said instrument secured to the armature coil and rotating in a plane extending through said coil, said instrument parts as assembled having an axial length no greater than the length of the armature coil.

CLEMENT Ff TAYLOR. CLYDE W. MCCARTY. ALLEN G. STIMSON.

REFERENCES CITED The following references are of record in the ille of this patent:

UNITED sTATEs PATENTS N ame Date 1,927,346 Lawrence Sept. 19, 1933 1,969,512 Johnson Aug. '1, 1934 2,139,997 Carson Dec. 13, 1938 2,228,424 Tonnies Jan. 14. 1941 FOREIGN PATENT Number Country l Date 400,651 Great Britain Oct. 30, 1933 514,349

Great Britain Jan. 21, 1938 

